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Allen Test

Topic Overview

The blood supply to your hand normally comes from two
arteries: the radial artery and the ulnar artery. Before drawing blood for an
arterial blood gas test, your health professional will make sure that both
arteries are open and working correctly. A procedure called the Allen test may
be used to find out if the blood flow to your hand is normal.

For
the Allen test, the health professional drawing your blood will apply pressure
to the arteries in your wrist for several seconds. This will stop the blood
flow to your hand, and your hand will become cool and pale. Blood is then
allowed to flow through the artery that will not be used to collect the blood
sample. This is usually the ulnar artery, which is found on the outer (little
finger side) of your wrist. Arterial blood gases are usually taken from the
radial artery, which is found on the inner (thumb side) of the wrist.

Allen test

Normal (positive)

Your hand quickly becomes warm and returns to
its normal color. This means that one artery alone will be enough to supply
blood to your hand and fingers.

Abnormal (negative)

Your hand remains pale and cold. This means that
one artery is not enough to supply blood to your hand and fingers. Blood will
not be collected from an artery in this hand.

If your hand remains pale and cold, the
Allen test will then be performed on your other hand. If your other hand also
remains pale, the blood often will be collected from another artery, usually in
the groin or elbow crease.

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